Off-resonance blurring is a common problem for radial ultra-short echo time sequences such as zero echo time (“ZTE”) and sweep imaging with Fourier transform (“SWIFT”). One efficient and simple solution to address this off-resonance blurring is to use higher readout bandwidth. However, these pulse sequences need fast switching of transmit and receive (T/R) mode to achieve higher bandwidth, which confronts a severe limitation in clinical MRI scanners.
Pointwise Encoding Time reduction with Radial Acquisition (“PETRA”) overcomes the T/R switching limitation by combining ZTE with a single point imaging (“SPI”) acquisition. In ZTE, slow T/R switching results in missing k-space center points. Thus, PETRA acquires the missing k-space center points with additional SPI acquisitions.
However, while PETRA alleviates the requirement on fast T/R switching, the use of higher bandwidth increases the number of missing k-space center points. In general, the number of missing k-space center points is proportional to the cube of the readout bandwidth. Therefore, higher bandwidth with slow T/R switching results in long additional SPI acquisition times. Another drawback of using higher bandwidth in PETRA is an increase of radiofrequency (“RF”) pulse peak power and specific absorption rate (“SAR”) because the higher readout bandwidth results in an increase of excitation bandwidth. The increase in RF pulse peak power and SAR with higher bandwidth limits the available flip angles that can be safely used in PETRA.
Thus, there remains a need for an ultra-short echo time pulse sequence that overcomes the off-resonance blurring issues of ZTE and SWIFT, without the limitations that PETRA has on receiver bandwidth, scan duration, and available flip angles.